The invention relates to the use of additive combinations which are suitable for avoiding undesired skin formation in lacquer systems that dry by oxidation, and for improving the through-drying of the lacquer films.
The additives used according to the invention are characterised by the presence of primary, secondary or tertiary aliphatic amines and/or mixtures thereof in combination with dicarbonyl compounds.
The invention is in the field of colourless and pigmented lacquers and paints which dry by oxidation and are based on oils which dry by oxidation, alkyd resins, epoxy esters and other, refined oils which dry by oxidation, as well as in the field of printing inks. It relates to novel additives which are capable of delaying skin formation in the above-mentioned lacquer systems. Such additive systems are additionally capable of improving the through-drying and the flow of the lacquer systems.
Oils and binders which crosslink by oxidation by the action of oxygen (preferably atmospheric oxygen) by means of the addition of drying agents, for example metal carboxylates of transition metals, and as a result form a solid binder film may form a skin on their surface when they are stored in open or closed containers. That crosslinking, which takes place even before the product is actually used, is undesirable to a large degree and should therefore be avoided, because it renders handling of the lacquer, for example, more difficult and impairs uniform distribution of the siccatives. The accumulation and incorporation of the siccatives that are necessary for drying in the lacquer skin that forms can lead to significant delays in the drying of the lacquer on application.
Skin formation is also disadvantageous and therefore undesirable in the case of the applied lacquer film. Too rapid drying of the lacquer surface prevents uniform through-drying of the lower film layers by shielding them from the oxygen that is necessary for drying, which is unable sufficiently to penetrate the lacquer film and be distributed therein owing to too rapid drying at the surface. Disturbances in the flow of the lacquer film, for example adhesion problems or films which are not sufficiently hard, may result.
It is therefore state of the art to add to the lacquer organic substances which inhibit the reaction of the siccative metal with (atmospheric) oxygen. This can be effected both by binding of the oxygen and by complexing of the siccative metal. That object is achieved in the art mostly by the addition of oximes (especially butanoneoxime) or suitable phenolic compounds. A list of such known compounds will be found, for example, in H. Kittel “Lehrbuch der Lacke und Beschichtungen”, Colomb Verlag 1976; J. Bieleman “Lackadditive” Wiley VCH 1998; Römpp Lexikon “Lacke und Druckfarben”, Thieme Verlag 1998. In WO 00/11090, pyrazoles are recommended for that purpose.
However, phenolic anti-skinning agents markedly delay the onset of drying, so that they are suitable only for specific lacquer formulations. Oximes, on the other hand, such as, for example, methyl ethyl ketoxime or butyraldoxime, delay the onset of drying only slightly owing to their volatility. The most important disadvantage of oximes, which are nowadays used on a large scale, is their toxicity. For example, in a long-term inhalation study on rats and mice, an increased occurrence of liver tumours was observed following exposure to butanoneoxime, on the basis of which the German MAK (maximum concentration at the workplace) commission has classified the substance as a category 2 carcinogen (MAK-Liste 1997). For the user, the result is that complicated personal protective measures must be maintained when processing lacquers containing oximes as anti-skinning agents.
Accordingly, the object of the present invention was to provide anti-oxidants (anti-skinning agents) which prevent skin formation on lacquers over a long period of time and which do not delay the onset of drying, or delay it only very slightly. In addition, the resulting film hardnesses should not be adversely affected, and the products should have no disadvantageous toxicological properties.
A further object of the present Application was to prepare anti-skinning agents which can be incorporated into many different lacquers that dry by oxidation and which, on the basis of their physical properties, can be used without difficulty and widely in corresponding lacquer formulations.